Collenchyma cells form the principal medicinal tissue of stems of herbs, petioles of leaves, etc. In certain herbs the collenchyma forms several of the outer layers of the cortex of the stem. In motherwort, horehound, and in catnip the collenchyma cells occur chiefly at the angles of the stem. In motherwort (Plate 29, Fig. B) there are twelve bundles, one large bundle at each of the four angles, and two small bundles, one on either side of the large bundle. In catnip (Plate 29, Fig. A) there are four large masses, one at each angle of the stem.

Collenchyma cells differ from parenchyma cells in a number of ways: first, the cell cavity is smaller; secondly, the walls are thicker, the greater amount of thickening being at the angles of the cells - that is, the part of the cell wall which is opposite the usual intercellular space of parenchyma cells, while the wall common to two adjoining cells usually remains unthickened. In horehound stem (Plate 30, Fig. 2) the thickening is so great at the angles that no intercellular space remains. In the side column of motherwort stem (Plate 30, Fig. 1) the thickening between the cells has taken place to such an extent that the cell cavities become greatly separated and arranged in parallel concentric rows.

The collenchyma of the outer angle of motherwort stem (Plate 30, Fig. 3) is greatly thickened at the angles. There are no intercellular spaces between the cells, and cell cavity is usually angled in outline instead of circular, as in the cells of horehound. In certain plants intercellular spaces occur between the cells, and the walls are striated instead of being non-striated, as in the stems of horehound, motherwort, and catnip.

Collenchyma cells retain their living contents at maturity. Many collenchyma cells, particularly of the outer layers of bark and the collenchyma of the stems of herbs, contain chlorophyll.

A. Diagrammatic sketch of the cross-section of catnip stem.

Plate 29. A. Diagrammatic sketch of the cross-section of catnip stem (Nepeta cateria, L.). I. Collenchyma occurring at the four angles of the stem.

B. Diagrammatic sketch of the cross-section of motherwort stem.

B. Diagrammatic sketch of the cross-section of motherwort stem (Leonurus cardiaca, L.). 1, 2, 3. Twelve masses of collenchyma tissue occurring at the four sides of the stem.

Collenchyma Cells.

Plate 30. Collenchyma Cells.

1. Cross-section of a side column of the collenchyma of motherwort stem (Leonurus cardiaca, L.).

2. Cross-section of the collenchyma of horehound stem (Marrubium vulgare, L.).

3. Cross-section of the collenchyma of the outer angle of motherwort stem.

The walls of collenchyma consist of cellulose.